Circuit for measuring photo-current level in fiber optic links

ABSTRACT

A fiber optic transceiver or receiver having circuitry for detecting signal strength of a photo-diode therein is disclosed. In one embodiment, the photo-current generated by the photo-diode is provided to a pre-amplifier circuit. The pre-amplifier circuit generates differential signals including a first signal and a second signal whose difference is representative of the detected photo-current. The fiber optic receiver further includes a post-amplifier circuit that is coupled to the pre-amplifier circuit to receive the differential signals. In one embodiment, the post-amplifier circuit includes circuitry to accurately derive a signal strength of the photo-current from the differential signals.

The present application claims priority to, under 35 U.S.C. 119(e), United States Provisional Patent Application bearing serial No. 60/357,608, filed Feb. 14, 2002, which is incorporated herein by reference.

BRIEF DESCRIPTION OF THE INVENTION

The present invention relates generally to signal detectors, and particularly to optical signal strength detectors.

BACKGROUND OF THE INVENTION

It is useful to measure the received optical power in fiber optic links in order to determine the integrity of the signal path and terminal devices. The photo-current in the photo-diode is proportional to the received power. Hence, one way of measuring received optical power is to measure the photo-current level.

Typical optoelectronic receivers utilize a photo-detector which is integrated in the same TO (Transistor Outline) package as the pre-amplifier. Most TO packages have only four pins. Two of the pins are connected to receive a differential signal from the pre-amplifier, one of the pins receives a power supply voltage, and one of the pins is for ground. Since all four pins of the TO packages each have a defined function, there is no separate lead available to output the photo-current. A fifth pin would not be an ideal solution because space is at a premium in TO packages.

Several prior art signal detectors measure the photo-current level without the use of a fifth pin. At low signal amplitudes (e.g., at 0.5 mV or less), however, these signal detectors exhibit a significant loss of gain—and thus accuracy. This loss of gain at low signal amplitudes is caused by a breakdown of the switch-like behavior of these signal detectors.

SUMMARY OF THE INVENTION

The present invention provides a more accurate method and apparatus for measuring the photo-current of a photo-diode in a fiber optic receiver or transceiver. In particular, the photo-current level is derived from the differential output of a pre-amplifier circuit.

An aspect of the present invention includes a voltage difference detector circuit that comprises first, second, third, fourth, fifth, and sixth circuit elements. The first circuit element is configured to receive a first voltage of a differential voltage pair. The second circuit element is configured to receive a second voltage of the differential voltage pair. The third circuit element connects the first circuit element to the second circuit element and is configured such that a first current proportional to a difference between the first voltage and the second voltage passes through the third circuit element. The fourth circuit element is coupled to the first circuit element and to the fifth circuit element and is configured to be affected by the first current such that a second current substantially equal in amplitude to the first current passes through the fourth circuit element to the fifth circuit element when the second voltage is greater than the first voltage. The sixth circuit element is coupled to the second circuit element and to the fifth circuit element and is configured to be affected by the first current such that a third current substantially equal in amplitude to the first current passes through said sixth circuit element to the fifth circuit element when the first voltage is greater than the second voltage. As a result, the difference between the first voltage and the second voltage may be determined by reference to the second current and the third current that flow to the fifth circuit element.

Another aspect of the present invention also includes a voltage difference detector circuit that comprises a voltage-to-current converter, a first current regulator, a second current regulator, and a current-to-voltage converter. The voltage-to-current converter is configured to 1) receive a first voltage and a second voltage of a differential voltage pair, 2) convert a difference between the first voltage and the second voltage to a first current, 3) draw a second current from the first current regulator substantially equal to a current offset plus the first current, and 4) draw a third current from the second current regulator substantially equal to a current offset minus the first current. The first current regulator is configured to produce at least a first amount of current and to produce a fourth current that flows to the current-to-voltage converter when the first amount of current is greater than the second current—the fourth current is substantially equal to a difference between said second current and said first amount of current. The second current regulator is configured to produce at least a second amount of current and to produce a fifth current that flows to the current-to-voltage converter when the second amount of current is greater than the third current—the fifth current is substantially equal to a difference between the third current and the second amount of current. The current-to-voltage converter is configured to convert the fourth current to a third voltage and to convert the fifth current to a fourth voltage. The third voltage and the fourth voltage are proportional to a difference between the first voltage and the second voltage.

Still another aspect of the present invention also includes a signal detector circuit that comprises a first portion, a second portion, a first resistor, and a second resistor. The first portion is coupled to receive a first signal. The second portion is coupled to receive a second signal that is complementary to the first signal. The first resistor is coupled to the first portion and the second portion. The second resistor is coupled to the first portion and the second portion. A current generated in response to a difference between the first signal and the second signal and flowing across the first resistor causes a corresponding current to flow across the second resistor to produce a potential difference that is representative of said difference between the first and second signals.

Yet another aspect of the present invention also includes a signal detector circuit. The signal detector circuit includes a first left transistor with a first emitter terminal coupled to a first resistor and a first left constant current source, a first collector terminal coupled to a left current drain, a second left constant current source, and a third left current source, and a first base terminal that receives a first signal, which fluctuates between a maximum voltage and a minimum voltage. The signal detector circuit also includes a first right transistor with a second emitter terminal coupled to the first resistor and a first right constant current source, a second collector terminal coupled to a right current drain, a second right constant current source, and a third right current source, and a second base terminal that receives a second signal that is complementary to the first signal. The signal detector circuit further includes a second resistor coupled to the left current drain and the right current drain. A current generated in response to a difference between the first signal and the second signal and flowing across the first resistor causes a corresponding current to flow across the second resistor to produce a potential difference that is representative of a difference between the first signal and the second signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present invention will be more readily apparent from the following description and appended claims when taken in conjunction with the accompanying drawings, in which:

FIG. 1A is a block diagram of an optical communication system according to an embodiment of the present invention;

FIG. 1B is a graph of the light intensity of an exemplary light signal sent by the transmitter over a period of time;

FIG. 1C is a graph of the signal strength of an exemplary electrical signal produced by a photo-diode over a period of time;

FIG. 1D is a graph of the signal strength of exemplary differential signals produced by a first stage circuit over a period of time;

FIG. 2A is a diagram of a signal detector circuit in accordance with the present invention;

FIG. 2B is a graph of the voltage at points A_(L) and B_(L) of the signal detector circuit of FIG. 2A;

FIG. 2C is a graph of the voltage at points A_(R) and B_(R) of the signal detector circuit of FIG. 2A;

FIG. 2D is a graph of the voltage across a first resistor of the signal detector circuit of FIG. 2A;

FIG. 2E is a graph of the voltage across a second resistor of the signal detector circuit of FIG. 2A;

FIG. 2F is a diagram of another signal detector circuit in accordance with the present invention; and

FIG. 2G is a graph of the voltage across the second resistor of the signal detector circuit of FIG. 2F.

DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described. It will be appreciated that in the development of any such embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

FIG. 1A shows a block diagram of an optical communication system 100 according to one embodiment of the present invention. The communication system 100 includes an optoelectronic transmitter/transceiver 102 that transmits a light signal 104 with an associated frequency and light intensity. The transmitter 102 typically includes a laser and optics (not illustrated) to transmit the light signal 104. A light intensity graph 106 included in FIG. 1B shows the light intensity of an exemplary light signal 104 sent by the transmitter 102 over a period of time. The light intensity corresponds to the voltage of the light signal 104. The peak-to-peak amplitude or signal strength of the light signal 104 is denoted on the light intensity graph 106 by the symbol 2Δ.

The communication system 100 also includes an optoelectronic receiver/transceiver 110 that receives the light signal 104 from the transmitter 102.

The receiver 110 contains a photo-diode 112, a first stage circuit 118 (e.g., a Pre-Amplifier Circuit), and a second stage circuit 124 (e.g., a Post-Amplifier Circuit). In an exemplary implementation, the photo-diode 112 and the first stage circuit 118 are integrated into a Transistor Outline (TO) package 119, which is typically part of the optoelectronic receiver/transceiver 110.

The photo-diode 112 receives the light signal 104 from the transmitter 102 and converts it to an electrical signal 114. The electrical signal 114 has an associated signal strength corresponding to the voltage of the electrical signal 114. A signal intensity graph 116 included in FIG. 1C shows the signal strength of an exemplary electrical signal 114 produced by the photo-diode 112 over a period of time. The peak-to-peak amplitude or signal strength of the electrical signal 114 is denoted on the signal intensity graph 116 as 2Δ′. The signal strength 2Δ′ of the electrical signal 114 is proportional to the signal strength 2Δ of the light signal 104. This proportionality is a function of the photo diode's 112 scaling or loss factors.

The first stage circuit 118 receives the electrical signal 114 from the photo-diode 112 and produces amplified differential signals 120. The amplified differential signals 120 consist of a positive pre-amplifier output and a negative pre-amplifier output. The amplified differential signals 120 have a signal strength that corresponds to the voltage thereof. A signal intensity graph 122 included in FIG. 1D shows the signal strength of exemplary differential signals 120 produced by the first stage circuit 118 over a period of time. In FIG. 1D, the positive pre-amplifier output corresponds to the dashed line and the negative pre-amplifier output corresponds to the solid line. As shown by the signal intensity graph 122, the voltage of the positive pre-amplifier output is at a maximum (e.g., at MΔ′) when the voltage of the negative pre-amplifier output is at a minimum (e.g., at −MΔ′) and vice versa. In other words, the amplified differential signals 120 are nominally equal in amplitude, but 180 degrees out of phase.

The peak-to-peak amplitude or signal strength of the differential signals 120 is represented on the signal intensity graph 122 as 2MΔ′, where M corresponds to the amplification factor of the first stage circuit 118. The signal strength 2Δ of the light signal 104 may be derived from the signal strength 2MΔ′ of the differential signals 120 by first determining the signal strength 2Δ′ of the electrical signal 114 using the amplification factor M of the first stage circuit 118 and then determining the signal strength 2Δ of the light signal 104 using the scaling or loss factors of the photo diode 112.

The second stage circuit 124 receives the differential signals 120 from the first stage circuit 118 for further processing. The present invention provides particular circuit designs for use as part of the second stage circuit 124 to determine the signal strength 2MΔ′ of the differential signals 120.

Signal Detector Circuit

FIG. 2A shows a general circuit layout for a signal detector circuit 200 for use in or as the second stage circuit 124 of FIG. 1A in accordance with an embodiment of the present invention. As shown in FIG. 2A, the signal detector circuit 200 includes a first resistor 206 with a resistance value of R1, a second resistor 208 with a resistance value of R2, a first left transistor 202 (i.e., a first transistor positioned to the left of the first resistor 206 in FIG. 2A), a first right transistor 204 (i.e., a first transistor positioned to the right of the first resistor 206 in FIG. 2A), a first left current source 212 producing a current I1 _(L), a first right current source 214 producing a current I1 _(R), a second left transistor 216, a second right transistor 218, a third left resistor 220 with a resistance value of R3 _(L), a third right resistor 222 with a resistance value of R3 _(R), a fourth left resistor 224 with a resistance value of R4 _(L), a fourth right resistor 226 with a resistance value of R4 _(R), a third left transistor 228, a third right transistor 230, a fifth left resistor 232 with a resistance value of R5 _(L), a fifth right resistor 234 with a resistance value of R5 _(R), a sixth left resistor 236 with a resistance value of R6 _(L), a sixth right resistor 238 with a resistance value of R6 _(R), a second left current source 240 producing a current I2 _(L), and a second right current source 242 producing a current I2 _(R).

On the left side of the signal detector circuit 200, the emitter terminal of the first left transistor 202 is coupled to the first resistor 206 and the first left current source 212. The collector terminal of the first left transistor 202 is coupled to the source terminal of the second left transistor 216, the source terminal of the third left transistor 228, and the second left current source 240. The base terminal of the first left transistor 202 is coupled to the first stage circuit 118 (FIG. 1A) and receives a first of the differential signals 120 produced by the first stage circuit 118.

The gate terminal of the second left transistor 216 is coupled to the third and fourth left resistors 220, 224. The third left resistor 220 is also coupled to a power source and the fourth left resistor 224 is also coupled to circuit ground. The drain terminal 213 of the second left transistor 216 is coupled to the second resistor 208 and the drain terminal 211 of the second right transistor 218. The second resistor 208 is also coupled to circuit ground.

The gate terminal of the third left transistor 228 is coupled to the fifth and sixth left resistors 232, 236. The fifth left resistor 232 is also coupled to a power source and the sixth left resistor 236 is also coupled to circuit ground. The drain terminal 215 of the third left transistor 228 is coupled to a power source. The source terminal of the third left transistor 228 is coupled to the source terminal of the second left transistor 216 and coupled to the second left current source 240. Thus, the third left transistor 228 acts as a third left power source.

On the right side of the signal detector circuit 200, the emitter terminal of the first right transistor 204 is coupled to the first resistor 206 and the first right current source 214. The collector terminal of the first right transistor 204 is coupled to the source terminal of the second right transistor 218, the source terminal of the third right transistor 230, and the second right current source 242. The base terminal of the first right transistor 204 is coupled to the first stage circuit 118 (FIG. 1A) and receives a second of the differential signals 120 produced by the first stage circuit 118.

The gate terminal of the second right transistor 218 is coupled to the third and fourth right resistors 222, 226. The third right resistor 222 is also coupled to a power source and the fourth right resistor 226 is also coupled to circuit ground. The drain terminal 211 of the second right transistor 218 is coupled to the second resistor 208 and the drain terminal 213 of the second left transistor 216.

The gate terminal of the third right transistor 230 is coupled to the fifth and sixth right resistors 234, 238. The fifth right resistor 234 is also coupled to a power source and the sixth right resistor 238 is also coupled to circuit ground. The drain terminal 217 of the third right transistor 230 is coupled to a power source. The source terminal of the third right transistor 230 is coupled to the source terminal of the second right transistor 218 and coupled to the second right current source 242. Thus, the third right transistor 230 acts as a third right power source.

Preferably, the first left transistor 202 and first right transistor 204 are bipolar junction transistors (BJTs) and the second left transistor 216, second right transistor 218, third left transistor 228, and third right transistor 230 are field effect transistors (FETs). The use of these transistors, however, should not be read as a limitation of the invention as other transistor types or combinations may be used without departing from the scope of the present invention.

With respect to the current I_(EL), which is the current flowing out of the emitter of the first left transistor 202, it is actually equal to the current I_(CL), which is the current flowing into the collector of the first left transistor 202, plus the current flowing into the base terminal of the first left transistor 602. But in preferred embodiments of the present invention, the current I_(EL) is substantially equal to the current I_(CL) because the current I_(CL) is much greater than the current flowing into the base terminal of the first left transistor 202. Similarly, the current I_(ER), which is the current flowing out of the emitter of the first right transistor 204, is substantially equal to the current I_(CR), which is the current flowing into the collector of the first left transistor 204, because the current I_(CR) is much greater than the current flowing into the base terminal of the first right transistor 204. So for purposes of describing the present invention, the currents I_(EL) and I_(CL) and the currents I_(ER) and I_(CRL), respectively, are assumed to be identical. And the values of the currents I1 _(L), I1 _(R), I2 _(L), and I2 _(R) produced by the first left current source 212, the first right current source 214, the second left current source 240, and the second right current source 242, respectively, are all approximately equal.

Further, the signal detector circuit 200 is preferably configured such that the power sources, resistors, and circuit ground connected directly or indirectly to the gate terminals of the second and third left transistors 216, 228 and the second and third right transistors 218, 230, respectively, turn these transistors on and off (e.g., enable the flow of current through the source and drain terminals of these transistors) instead of controlling the flow of current through the source and drain terminals of these transistors so that it corresponds to the voltage at the respective gate terminals of these transistors. So significant current flows through the source and drain terminals of these transistors only when “pulled” or “pushed” by other aspects of the signal detector circuit 200.

The arrows on the source terminals of the second and third left transistors 216, 228 and the second and right transistors 218, 230 indicate the only direction in which current may flow through these terminals. So if the amplitude of the current I_(CL) is less than the amplitude of the current I2 _(L), which is produced by the second left current source 240, current flows into and out of the source and drain terminals, respectively, of the second left transistor 216. Conversely, if the amplitude of the current I_(CL) is greater than the amplitude of the current I2 _(L), current flows from the source terminal of the third left transistor 228.

Similarly, if the amplitude of the current I_(CR) is less than the amplitude of the current I2 _(R), which is produced by the second right current source 242, current flows into and out of the source and drain terminals, respectively, of the second right transistor 218. Conversely, if the amplitude of the current I_(CR) is greater than the amplitude of the current I2 _(R), current flows from the source terminal of the third right transistor 230.

Additionally, the voltage drop across the respective collector and emitter terminals of the first left and right transistors 202, 204 is preferably greater than or equal to 0.5 volts. This ensures that the first left and right transistors 202, 204 are turned on. A voltage drop greater than or equal to 0.5 volts is preferably accomplished with standard transistor biasing techniques known in the art.

Persons skilled in the art recognize that when in the active region, the current that flows through the collector and emitter terminals of a transistor is limited by the current received at the base terminal of these transistors and/or the voltage drop across the base terminal and the emitter terminal of these transistors. In other words, such inputs to a transistor may enable a certain amount of current to flow through a transistor, but this amount of current only flows if connected circuitry makes this current available. In embodiments of the present invention, the amplitude of the current produced by the constant current sources 212, 214, 240, 242 and the resistive value of the first resistor are preferably selected so that the current that flows through the collector and emitter terminals of the first left and right transistors 202, 204 is only a function of the voltage drop across the first resistor 206 and not the gain of these transistors.

Operation of the Signal Detector Circuit

In the course of the operation of the signal detector circuit 200, the base terminal of the first left transistor 202 receives one of the differential signals 120 produced by the first stage circuit 118 and the base terminal of the first right transistor 204 receives the other differential signal 120 produced by the first stage circuit 118. Again, the voltages received at the base terminals of the first left transistor 202 and the first right transistor 204, respectively, are nominally equal in amplitude, but 180 degrees out of phase. So if the differential signal 120 received at the base terminal of the first left transistor 202 (i.e., at point A_(L)) has a voltage of K+MΔ′, the differential signal 120 at the base terminal of the first right transistor 204 (i.e., at point A_(R)) has a voltage of K−MΔ′ and vice versa. K is preferably a DC voltage offset sufficiently high enough to at least ensure that the first left transistor 202 and the first right transistor 204 are turned on regardless of the value of the differential signals 120. More specifically, the offset voltage across the base and emitter of a typical transistor (e.g., the first left transistor 202 and the first right transistor 204) is usually 0.6 or 0.7 volts. Persons skilled in the art recognize that the voltage at the base of a typical transmitter (e.g., K+MΔ′ to K−MΔ′) must be greater than the offset voltage in order for the transistor to be turned on. Numerous circuits (not illustrated) known in the art may be used to add the DC offset voltage K to the differential signals 120 produced by the first stage circuit 118 without departing from the scope of the present invention.

The voltage at point A_(L) over time is shown in FIG. 2B as a solid line. As shown in FIG. 2B, the differential signal 120 received at point A_(L) has a peak-to-peak amplitude of 2MΔ′ and fluctuates between a maximum voltage of K+MΔ′ and a minimum voltage of K−MΔ′. The voltage at point A_(R) over time is shown in FIG. 2C as a solid line. As shown in FIG. 2C, the differential signal 120 received at point A_(R) also has a peak-to-peak amplitude of 2MΔ′ and fluctuates between a maximum voltage of K+MΔ′ and a minimum voltage of K−MΔ′.

When the voltage at point A_(L) is equal to K+MΔ′, the voltage at the emitter terminal of the first left transistor 202 (i.e., at point B_(L)) is equal to (K+MΔ′−0.7V), 0.7V being a typical offset voltage across the base and emitter of a transistor. Similarly, when the voltage at point A_(R) is equal to K−MΔ′, the voltage at the emitter terminal of the first right transistor 204 (i.e., at point B_(R)) is equal to (K−MΔ′−0.7V). The voltage at points B_(L) and B_(R) are shown in FIGS. 2B and 2C, respectively, as dashed lines. As shown in FIG. 2B, the peak-to-peak voltage at point B_(L) over time is the same as the voltage at point AL but offset by 0.7V. Similarly, the peak-to-peak voltage at B_(R) over time is the same as the voltage at A_(R) but offset by 0.7V.

When the voltage at point B_(L) is equal to (K+MΔ′−0.7V) and the voltage at point B_(R) is equal to (K−MΔ′−0.7V), the voltage V_(R1) across the first resistor 206 is equal to (K+MΔ′−0.7V)−(K−MΔ′−0.7V), which is equal to 2MΔ′. When the voltage at point B_(L) is equal to (K−MΔ′−0.7V) and the voltage at point B_(R) is equal to (K+MΔ′−0.7V), the voltage V_(R1) across the first resistor 206 is equal to (K−MΔ′−0.7V)−(K+MΔ′−0.7V), which is equal to −2MΔ′. The voltage V_(R1) over time is shown in FIG. 2D and varies from a maximum voltage of 2MΔ′ to a minimum voltage of −2MΔ′. The current I_(R1) flowing through the first resistor 206, therefore, is equal to (V_(R1)/R1) and varies from (2MΔ′/R1) to (−2MΔ′/R1) over time.

The value of the current I_(EL) flowing out of the emitter terminal of the first left transistor 202 is equal to the current I1 _(L) produced by the first left current source 212 plus the current I_(R1) flowing through the first resistor 206. Therefore, when the voltage at point A_(L) is at a maximum voltage of K+MΔ′, the current I_(EL) is equal to (I1 _(L)+2MΔ′/R1). And when the voltage at point A_(L) is at a minimum voltage of K−MΔ′, the current I_(EL) is equal to (I1 _(L)+(−2MΔ′/R1)).

So when the voltage at point A_(L) is at a maximum voltage of K+MΔ′, current I_(CL), which is assumed to be equal to the current I_(EL), is (I1 _(L)+2MΔ′/R1). If the current I2 _(L) produced by the second left current source 240 is equal to I1 _(L), a current equal to (2MΔ′/R1) is pulled from the third left transistor 228 and no significant amount of current flows into the second left transistor 216.

And when the voltage at point A_(L) is at a minimum voltage of K−MΔ′, current I_(CL) is (I1 _(L)+(−2MΔ′/R1)). If the current I2 _(L) produced by the second left current source 240 is equal to I1 _(L), a current equal to (2MΔ′/R1) is pushed through the second left transistor 216 and no significant amount of current flows through the third left transistor 228.

In other words, when the current flowing through the first left transistor 202 is less than the current produced by the second left current source 240, the excess amount of the current produced by the second left current source 240 flows through the second left transistor 216. And when the current flowing through the first left transistor 202 is greater than the current produced by the second left current source 240, the shortfall of current flows through the third left transistor 228 and combines with the current produced by the second left current source 240.

As noted above, current only flows through the transistors illustrated in FIG. 2A in the direction of the corresponding arrows. So when current flows (i.e., is pushed) through the second left transistor 216, all of this current flows through the second resistor 208—none of-this current flows through the second right transistor 218. The voltage drop across the second resistor 208 when the voltage at point A_(R) is at a minimum voltage of K−MΔ′, therefore, is equal to (2MΔ′/R1)*R2.

The first and second left current sources 212, 240, therefore, are preferably configured to ensure that 1) a positive current always flows through the first left transistor 202, a requirement for the first left transistor 202 to remain on and 2) a current proportional to the differential signals 120 flows through the second left transistor 216 when the current I2 _(L) exceeds the current I_(CL). To do so, these current sources preferably each produce a current greater than (2MΔ′/R1).

The value of the current I_(ER) flowing out of the emitter terminal of the first right transistor 204 is equal to the current I1 _(R) produced by the first right current source 214 minus the current I_(R1) flowing through the first resistor 206. Therefore, when the voltage at point A_(R) is at a minimum voltage of K−MΔ′, the current I_(ER) is equal to (I1 _(R)+(−2MΔ′/R1)). And when the voltage at point A_(R) is at a maximum voltage of K+MΔ′, the current I_(ER) is equal to (I1 _(R)+2MΔ′/R1)).

So when the voltage at point A_(R) is at a minimum voltage of K−MΔ′, current I_(CR), which is assumed to be equal to the current I_(ER), is (I1 _(R)+(−2MΔ′/R1)). If the current I2 _(R) produced by the second right current source 242 is equal to I1 _(R), a current equal to (−2MΔ′/R1) is pushed through the second right transistor 218 and no significant amount of current flows through the third right transistor 230.

And when the voltage at point A_(R) is at a maximum voltage of K+MΔ′, current I_(CR) is (I1 _(R)+2MΔ′/R1). If the current I2 _(R) produced by the second right current source 242 is equal to I1 _(R), a current equal to (2MΔ′/R1) is pulled from the third right transistor 230 and no significant amount of current flows into the second right transistor 218.

In other words, when the current flowing through the first right transistor 204 is greater than the current produced by the second right current source 242, the shortfall of current flows through the third right transistor 230 and combines with the current produced by the second right current source 242. And when the current flowing through the first right transistor 204 is less than the current produced by the second right current source 242, the excess amount of the current produced by the second right current source 242 flows through the second right transistor 218.

When current flows (i.e., is pushed) through the second right transistor 218, all of this current flows through the second resistor 208—none of this current flows through the second left transistor 216. The voltage drop across the second resistor 208 when the voltage at point A_(R) is at a minimum voltage of K−MΔ′, therefore, is equal to (2MΔ′/R1)*R2.

The first and second right current sources 214, 242, therefore, are preferably configured to ensure that 1) a positive current always flows through the first right transistor 204, a requirement for the first right transistor 204 to remain on and 2) a current proportional to the differential signals 120 flows through the second right transistor 218 when the current I2 _(R) exceeds the current I_(CR). To do so, these current sources preferably each produce a current greater than (2MΔ′/R1).

So regardless of which differential signal is at a minimum voltage of K−MΔ′, the voltage across the second resistor 208 (i.e., the voltage V_(R2)) is equal to (2MΔ′/R1)*R2. The ideal output of the signal detector circuit 200 or the voltage drop, V_(R2), across the second resistor 208 is illustrated in FIG. 2E.

Since the values of R1 and R2 are known, the signal strength 2MΔ′ of the differential signals 120 can be determined from the voltage V_(R2) across the second resistor 208. As stated above, the signal strength 2Δ of the light signal 104 may be derived from the signal strength 2MΔ′ of the differential signals 120 by first determining the signal strength 2Δ′ of the electrical signal 114 using the amplification factor M of the first stage circuit 118 and then determining the signal strength 2Δ of the light signal 104 using the scaling or loss factors of the photo diode 112.

In some embodiments, the voltage drop across the second resistor 208 is input to, for example, an A/D converter 252—as illustrated in FIG. 2F. The output of the A/D converter may then be read by a microprocessor 254 or other device. The result can thus be read by an external system via a standard transceiver interface.

As stated above, output of the signal detector circuit 200 illustrated in FIG. 2E is an idealized output. In actuality, the voltage wave forms illustrated in FIGS. 2B-2D are not perfect square waves. Instead, a certain amount of time is required for the voltages to swing from a maximum amplitude to a minimum amplitude and vice versa. As a result, there will be periods of time in which neither of the voltages at the base terminals of-the first left transistor 202 and the first right transistor 204, respectively, are equal to the minimum voltage of K−MΔ′. During these periods of time, one of these voltage is between 0 and K−MΔ′ and the other is between 0 and K+MΔ′. So the current that flows through the second resistor 208, for these periods of time, varies between (2MΔ′/R1) to (−2MΔ′/R1). An exemplary, actual voltage drop, V_(R2), across the second resistor 208 is illustrated in FIG. 2G (this too is a somewhat idealized illustration as the actual voltage wave form is not likely entirely linear). As a result, a capacitor 250 may be included in some embodiments of the present invention in parallel with the second resistor 208, as also illustrated in FIG. 2F, in order to smooth out the wave form illustrated in FIG. 2G. In still other embodiments, the period of the differential signals is known so that the voltage V_(R2) is sampled only during periods of stability, which correspond to when the voltages at the base terminals of one or the other of the first left transistor 202 and the first right transistor 204 is equal to the minimum voltage of K−MΔ′.

But even with the slight errors described in the previous paragraph, the present invention represents an improvement over prior art signal detector circuits. This is due in part to the fact that the present invention does not rely upon the gain of the transistors included therein. Instead, the transistors are merely turned on to enable the flow of current. And the current across the second resistor 208 results from current across the first resistor 206, which is a function of the relative values of the differential signals 120, and the fact that current flows through transistors in only one direction regardless of temperature or method of manufacture. And any offset voltage variations of the transistors illustrated in FIG. 2A or FIG. 2F due to temperature are largely offset by equal changes in corresponding transistors (e.g., a change in the offset voltage of the first left transistor 202 is largely offset by an equal change in the offset voltage of the first right transistor 204).

While the present invention has been described with reference to a few specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications may occur to those skilled in the art having the benefit of this disclosure without departing from the inventive concepts described herein. 

1. An optoelectronic device, comprising: a photo-diode that generates a photo-current in response to an optical signal; a pre-amplifier circuit coupled to the photo-diode to detect the photo-current, the pre-amplifier circuit generating a differential output having a first signal and a second signal based on the photo-current; and a post-amplifier circuit coupled to the pre-amplifier circuit to receive the first signal and the second signal, the post-amplifier circuit comprising: a first portion coupled to receive the first signal; a second portion coupled to receive second signal; a first resistor coupled to the first portion and the second portion; a second resistor coupled to the first portion and the second portion; and wherein a current generated in response to a difference between the first signal and the second signal and flowing across the first resistor causes a corresponding current to flow across the second resistor to produce a potential difference that is representative of a difference between a maximum voltage and a minimum voltage of at least one of the first signal and second signal.
 2. The optoelectronic device of claim 1, wherein the first portion includes a first left transistor having: an first emitter terminal coupled to a first end of the first resistor and a first left constant current source; a first collector terminal coupled to a left current drain, a second left constant current source, and a third left current source; and a first base terminal that receives the first signal.
 3. The optoelectronic device of claim 2, wherein the second portion includes a first right transistor having: a second emitter terminal coupled to a second end of the first resistor and a first right constant current source; a second collector terminal coupled to a right current drain, a second right constant current source and a third right current source; and a second base terminal that receives the second signal.
 4. The optoelectronic device of claim 3, wherein the first left constant current source produces a current that is approximately equal to a current produced by the second left constant current source.
 5. The optoelectronic device of claim 3, wherein the first right constant current source produces a current that is approximately equal to a current produced by the second right constant current source.
 6. The optoelectronic device of claim 3, wherein the left current drain is configured such that a second current flows through said left current drain to the second resistor, said second current equal in magnitude to a difference between a current that flows into the first collector terminal and a current produced by the first left constant current source; the right current drain is configured such that a third current flows through said right current drain to the second resistor, said third current equal in magnitude to a difference between a current that flows into the second collector terminal and a current produced by the first right constant current source; and the second current and the third current together form the corresponding current.
 7. The optoelectronic device of claim 6, wherein the left current source is configured produce a fourth current when the current flowing into the first collector terminal is greater than a current produced by the first left constant current source, said fourth current equal in magnitude to a difference between a current that flows into the first collector terminal and the current produced by the first left constant current source the right current source is configured produce a fifth current when the current flowing into the second collector terminal is greater than a current produced by the first right constant current source, said fifth current equal in magnitude to a difference between a current that flows into the second collector terminal and the current produced by the first right constant current source. 